The present invention relates to a printer containing a printhead fixed on a scanning carriage, movement means for moving the scanning carriage and a receiving material with respect to one another in a main scanning direction and a sub-scanning direction substantially perpendicularly to the main scanning direction, and a guide means for guiding the scanning carriage in the main scanning direction.
A printer of this kind is known from U.S. Pat. No. 5,771,050. The movement means in this known printer comprises a roller for displacing a receiving material, for example a sheet of paper, with respect to the printhead in the sub-scanning direction. The scanning carriage is displaced in the main scanning direction over a set of guide rods using a conveyor belt. The printhead of this printer comprises a linear row of inkjet print elements extending in the sub-scanning direction so that a plurality of pixel rows can be printed during a scanning pass of the scanning carriage in the main scanning direction. In order to avoid visible printing artifacts as much as possible, the known printer is equipped with a means for adjusting, prior to each scanning pass, the position that the printhead and the receiving material occupy with respect to one another in the sub-scanning direction. Such a means comprises a sensor disposed on the printhead in order to detect a registration mark provided on the receiving material. In addition, suche means include actuators for changing the position of the printhead with respect to the receiving material in dependence on a signal delivered by the sensor. After each scanning pass, a registration mark is printed on the receiving material, using the printhead. The receiving material is then displaced in the sub-scanning direction. If the receiving material is displaced over the correct distance, the sensor will detect the registration mark at a predetermined location, thus ensuring that the distance between the new rows of pixels for printing and the printhead pixel rows is correct. However, if the receiving material is not moved over the correct distance, the sensor will detect a deviation between the measured location and the predetermined location of the registration mark. The consequence is that a signal is delivered to displace the printhead with respect to the scanning carriage in the sub-scanning direction until the printhead occupies the proper position with respect to the receiving material. A number of pixel rows are then printed in a following scanning pass, whereafter a new registration mark is printed on the receiving material.
This known printer has a significant disadvantage. Despite the accurate adjustment of the printhead with respect to the receiving material prior to each scanning pass, it has been found that visible printing artifacts can occur, and these printing artifacts are not the result of a breakdown or other incorrect operation of one of the print elements of the printhead. These artifacts may depend inter alia on the printer loading and the ambient conditions.
The object of the present invention is to provide a printer which avoids such artifacts as far as possible. To this end, the printer is provided with a displacement means for displacing the scanning carriage automatically with respect to the guide means during a movement of the scanning carriage in the main scanning direction. The displacement means can be any means suitable to displace the scanning carriage, such as, for example, means that displace the carriage in contact, such as mechanical means (e.g., with the aid of a spring, screw, push or pull element, by means of expansion or shrinkage of an actuator, etc.) or in non-contact such as by magnetic or other forces that do not need a tangible intermediate (e.g., gravitational forces, centrifugal forces, etc.). It has been found that a considerable proportion of printing artifacts can be avoided in this way. It is not completely clear where such printing artifacts originate, but closer examination shows that there are a number of sources of error which can be reduced to inaccurate guidance of the scanning carriage over the guide means during a movement of the scanning carriage. Thus it is possible that the scanning carriage may move with respect to the guide means, for example, become slightly skewed, during the scanning movement. It is also possible that the guide means itself, for example a guide rod or set of guide rods, is not completely straight. It has been found that guide rods of this kind, which initially are frequently very straight, may become curved to a greater or lesser degree after assembly in the printer, due to thermal or mechanical stresses. These curvatures may in some cases be of a permanent nature. The separate deviations are often minor but the result of all these deviations together is that pixels are perceptibly printed at incorrect positions so that visible printing artifacts may occur. By providing the printer with a displacement means for displacing the scanning carriage automatically, i.e. by the printer itself, during a scanning pass, with respect to the guide means, all these deviations can be corrected. As a result, pixels are printed in the correct position so that printing artifacts can be avoided. An additional advantage of the printer according to the present invention is that it requires much less accurate mechanical adjustments. Deviations in the scanning carriage guidance can always be corrected by displacing the scanning carriage. This results in a saving of production costs for the printer. In addition, use can be made of less accurate and hence cheaper guide means, for example a set of relatively thin rods, without these being supported by a support surface which has had after-treatment, as adequately known from the prior art.
In one preferred embodiment, the scanning carriage is displaceable in the sub-scanning direction. This embodiment has the advantage that it is possible to prevent all the printing artifacts which cannot be corrected by adjusting the print time of the print elements. In this way it is possible to print good images with a relatively simple printer.
In a further preferred embodiment, the scanning carriage is displaceable in such manner that it occupies a substantially constant position as considered in the sub-scanning direction. Since the position of the scanning carriage in the sub-scanning direction remains substantially constant during the movement of the scanning carriage in this embodiment, the position of the scanning carriage with respect to the receiving material also remains substantially constant in that direction. In this way the printhead fixed on the scanning carriage will also occupy a substantially constant position, at least in the sub-scanning direction, with respect to the receiving material, during the movement of the scanning carriage in the main scanning direction. This further prevents, in a simple manner, visible printing artifacts from forming in a printed image.
In one preferred embodiment, at least two heads are fixed on the scanning carriage. In this embodiment the invention offers the further advantage that correction for both heads takes place simultaneously simply by displacing the scanning carriage with respect to the guide means. This is a relatively inexpensive way of correcting in one operation a considerable number of deviations which can lead to printing artifacts, for all the printing heads fixed on the scanning carriage.
In a further preferred embodiment, the printer is adapted to print wide-format receiving materials. In the case of wide-format receiving materials, i.e. materials wider than A4, a scanning carriage is moved over a relatively considerable distance. As a result, the printer is more sensitive to deviations in the guide mechanism. Here the invention offers the specific advantage that even in the case of very wide formats, such as A0, these deviations have no adverse effects on the image quality, because they can be corrected using the displacement means.
In one embodiment, the displacement means comprises two actuators operatively connected to the scanning carriage and the guide means. In this embodiment, a printer according to the invention is obtained in a simple manner. If the two actuators are connected to two ends of the scanning carriage in the main scanning direction a maximum displacement of the scanning carriage is possible. The actuator may be any electromechanical aid, including electromagnetic, magnetostrictive, pneumatic or hydraulic actuators. Preferably, however, use is made of piezo-electric actuators, because they respond relatively quickly, have a stable deflection, and a substantially linear volt/deflection characteristic, which facilitates use in an electronic control circuit. Amongst the piezo-electric actuators, the ceramic multi-layer actuators (CMA""s) are particularly suitable because they have a considerable deflection and only require a small actuation voltage.
In a further embodiment, the scanning carriage is displaceable, using the displacement means, in a direction substantially perpendicular to the main scanning direction and the sub-scanning direction. In this way, deviations in a third direction (substantially perpendicular to the receiving material) can also be corrected. Correction in this way instead of adjusting the printing time of each of the print elements offers the advantage that the data processing in the printer is subjected to less loading. In addition, in this way irregularities in the receiving material, for example as a result of deviations in the feed surface or corrugations in the receiving material itself, can be corrected.
In a further preferred embodiment, a printhead is releasably fixed on the scanning carriage. In this embodiment, in which the printhead is replaceable, the invention offers the further advantage that no further adjustment means need to be integrated in the releasable fixing of the printhead. If, for example, the printhead has to be displaced in the sub-scanning direction because the receiving material has been incorrectly transported, this can be effected in principle also with the aid of the displacement means according to this invention. In principle it is possible to perform this and/or other small corrections with a correction means with which the position of the printhead with respect to the scanning carriage is controlled, for example in the manner known from the above-cited U.S. patent specification. The printer according to this latter embodiment, however, has the disadvantage that a complex adjustment means must be implemented for each printhead, and this is relatively expensive.